1. Technical Field
This invention relates generally to a system and method of controlling an internal combustion engine, and, in particular, to a system and method for controlling an air-to-fuel ratio of a gaseous-fueled internal combustion engine to a lean misfire limit of the engine.
2. Discussion of the Background Art
Owners and operators of industrial stationary engines have been concerned with both the efficiency of operation (i.e., fuel consumption of such engines) as well as emissions generated thereby for many years. In particular, the owners and operators of industrial stationary engines are subject to federal and state environmental regulations with respect to combustion products of such engines, such as NO.sub.x, CO.sub.2, and other emissions. Accordingly, there has been investigation into systems and methods for controlling gaseous-fueled engines to reduce emissions of certain types of combustion products. For example, conventional approaches for reduction of, for example, NO.sub.x emissions, are obtained by stoichiometric air-fuel (A/F) ratio operation in combination with non-selective catalytic reduction technology (NSCR). However, this approach uses a relatively increased amount of fuel.
Another approach taken in the art directed to optimizing efficiency and emissions has been to operate such engines at air-fuel ratios lean of stoichiometric. However, these approaches have shortcomings in producing reliable and effective operation.
As background, it is a known characteristic of gaseous-fueled internal combustion engines that they can be operated at air-fuel ratios lean of stoichiometric. Operation at these "lean" air-fuel ratios may not produce the output power called for; however, on the other hand, such operations may occur at air-fuel ratios not lean enough to be at a lean misfire limit of the engine. Thus, gaseous-fueled engines lose power (sometimes referred to as a loss of reserve power capacity) when operated at "lean" air-fuel ratios, even at air-fuel ratios substantially lean of stoichiometric, before operating erratically.
One problem generally with reciprocating engines employing conventional controls involves so-called "pumping losses." Known approaches for adjusting the air and fuel delivery for gaseous-fueled engines have a shortcoming in that even when operated at full rated power output, a throttle valve in the air and fuel delivery apparatus (e.g., carburetor) is not fully open. A undesirable trait of operating at less than wide-open throttle (WOT) is increased "pumping losses" (i.e., horsepower wasted by ingesting air through a flow limiting device, such as a partly closed throttle valve). Ostensibly this failure to operate the engine at wide-open throttle (WOT) is to allow the engine to have reserve power capacity in the event of control system drift. Since control system drift could not reliably be accounted for in known engine controls, it was therefore necessary to operate with such a reserve capacity.
As a result of pumping losses, fuel consumption of the engine is increased, thereby also increasing CO.sub.2 emissions. The increased amount of combusted fuel elevates combustion temperatures, thereby increasing oxides of nitrogen as a combustion product. In addition, combustion of the extra fuel elevates temperatures, which in turn increases thermal stress on various engine parts such as pistons, rings, valves, heads, exhaust manifolds, etc. This increases maintenance costs.
There are primarily two control strategies for the "lean" control of the air-fuel ratio of gaseous-fueled internal combustion engines: (i) open loop control (i.e., with no feedback information), and (ii) closed loop control (i.e., with feedback of a sensed variable indicative or otherwise a measure of the combustion process itself in some way, such as the use of an exhaust gas temperature parameter, an amount of oxygen in the exhaust parameter, a fuel pressure parameter, etc.). These two control strategies, as implemented in the art, have certain disadvantages.
Regarding known open loop air-fuel ratio control systems, a carburetor is typically used as the air and fuel delivery apparatus. The carburetor, due to the mechanics of the apparatus itself, fixes the ratio of air and fuel. In the open loop approach, the system is adjusted to an air-fuel ratio near the lean power loss/misfire limit. However, during operation, the degree of optimization actually realized varies depending on a variety of factors, such as changes in engine load, changes in relative humidity, changes in fuel characteristics (e.g., BTU per SCF, flame speed, hydrogen content, etc.), changes in atmospheric conditions, and the like. Inasmuch as open loop control does not use any feedback, the degree of air-fuel ratio "optimization" is left to the vagaries of system calibration drift, mechanical mixing limitations of the carburetor itself, mechanical degradation and changes in combustion variables such as ambient air conditions and changes in fuel characteristics. Maintaining an acceptable degree of air-fuel ratio optimization requires routine maintenance and calibration, which can become costly and invasive. In addition, there are reliability concerns. In particular, the engine can operate at air-fuel ratios rich of the lean power loss/misfire limit, but cannot operate at all at air-fuel ratios lean of the lean power loss/misfire limit. Therefore, when variations, due to the above factors, occur tending to lean the already predetermined "lean" air-fuel ratio provided to the engine, drastic drop offs of power output may be observed, with operation of the engine becoming erratic. In the worst case scenario, the engine may stop operating all together. Inasmuch as this situation is commercially unacceptable, the air-fuel ratio adjustment is configured so as to leave the air-fuel ratio richer than an optimal "lean" air-fuel ratio by a predetermined guard or safety margin. This safety margin is to allow for the above-described degradation in air-fuel control that could result in air-fuel ratios lean of the lean power loss/misfire limit being provided to the engine. The disadvantage of including this guard or safety margin is an increase in fuel consumption, which thereby directly increases CO.sub.2 emissions, as well as elevates combustion temperatures (which increases NO.sub.x. Known open loop control strategies have thus been found unsatisfactory in the foregoing respects.
Known closed loop control strategies have similar disadvantages. In known closed loop systems of the type including, for example, a carburetor, a sensor (e.g., such as an exhaust oxygen sensor or an exhaust temperature sensor) is used. The sensor provides a sensed variable signal that is indicative of the combustion process. The sensed variable signal is used in the control strategy to adjust the air-fuel ratio of the charge provided to the engine. However, one disadvantage of such a system is that the control of the air-fuel ratio can only be as accurate as the sensor output itself. Second, while such a sensor does measure a combustion-related event, it does not directly measure lean power loss/misfire, per se. A third disadvantage involves the fact that this approach is unable to detect (and thus track) factors such as ambient atmospheric changes, changes in fuel characteristics or traits (e.g., BTU per SCF, flame speed, hydrogen content, etc.), and sensor degradation/drift. A fourth disadvantage involves the fact that such sensor-based systems require regular calibration and maintenance checks, which increases maintenance costs. A fifth disadvantage is that such systems have an undesirable failure mode (i.e., sensors may fail in an undesirable fashion, rendering the engine inoperative). Sixth, as the engine itself changes with condition (e.g., wear), desired target values change (to which the system is controlled using the sensor output) and failure to make ongoing compensation to the predetermined "target" values will cause the controlled air-fuel ratio to deviate from the programmed optimum.
Therefore, to avoid reliability problems, such closed-loop systems are operated at less than an optimal air-fuel ratio by including a guard or safety margin. As noted above, including a "safety" margin generally results in increased fuel consumption, increased Co.sub.2 emissions, as well as elevated combustion temperatures (with the resulting undesirable effects thereof noted above). Moreover, many of the known closed-loop control systems employ a control action that is digital in nature (i.e., adjustments are made based on whether a sensor output is higher or lower than a threshold value). This "dithering" has in many instances an undesirable response characteristic.
Thus, there is a need to provide an improved system and method for controlling a gaseous-fueled stationary internal combustion engine that overcomes or minimizes one or more of the above-mentioned problems.